Air Interface/Device

In current 4G LTE (OFDMA) networks we require self-interference avoidance, which is strictly a timing based approach, and this causes higher power consumption and significant waste of control plane signaling resources. Interference resistant access schemes in 5G that could replace the currently used OFDM are under intensive evaluation. More efficient orthogonal schemes such as FBMC, UFMC or GFMD, which introduce a better use of the spectrum are excellent candidates for the upcoming 5G access waveform. Generally, orthogonal transmission can avoid self-interference and this leads to a higher system capacity. However, for rapid access of small payloads, the procedure to assign orthogonal resources to different users may require extensive signaling and lead to additional latency. Thus support for non-orthogonal access, as a complement to orthogonal access, is being considered as well. Examples include Non-Orthogonal Multiple Access (NOMA) and Sparse-Code Multiple Access (SCMA).

FBMC, Filter Bank Multi-Carrier

FBMC has gained a high degree of interest as a potential 5G waveform candidate. This modulation scheme provides many advantages.

FBMC has many similarities to CP-OFDM, (OFDM using a cyclic prefix which is used as the 4G waveform). Instead of filtering the whole band as in the case of OFDM, FBMC filters each sub-carrier individually. FBMC does not have a cyclic prefix and as a result it is able to provide a very high level of spectral efficiency.

The subcarrier filters are very narrow and require long filter time constants. Typically the time constant is four times that of the basic multicarrier symbol length and as a result, single symbols overlap in time. To achieve orthogonality, offset-QAM is used as the modulation scheme, so FBMC is not orthogonal with respect to the complex plane.

UFMC, Universal Filtered Multi-Carrier

This waveform can be considered as an enhancement of CP-OFDM. It differs from FBMC in that instead of filtering each subcarrier individually, UFMC splits the signal into a number of sub-bands which it then filters.

UFMC does not have to use a cyclic prefix, although one can be used to improve the inter-symbol interference protection.

GFDM, Generalized Frequency Division Multiplexing

GFDM is a flexible multi-carrier transmission technique which bears many similarities to OFDM.

The main difference is that the carriers are not orthogonal to each other. GFDM provides better control of the out-of-band emissions and this reduces the peak to average power ratio, PAPR. Both of these issues are the major drawbacks of OFDM technology.

Massive MIMO

MIMO has been deployed in LTE and LTE-Advanced networks, where the base station and user equipment use multiple antennas to increase link efficiency. Massive MIMO refers to the technique where the base station employs a much higher number of antennas that create localized beams towards each device. The gains in capacity are enormous but so are the technical challenges associated with this technology.

Massive MIMO is building upon previous research and development in phased array antennas that was principally developed for electronically steered radar systems. The basic concept is that an array of low gain and low directivity antennas are built, and then the phase relationship between the signals on each antenna carefully managed such that the composite signal from all the sub-antennas gives a high gain and directional beam that is controlled by electronically adjustable phase shifters. This will lead to much higher data capacity in a massive MIMO cell, due to the ability to synthesize many separate simultaneous data paths to individual users.

The basic physics principles for massive MIMO are now proven, and experimental systems are being deployed.